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applied to the measured one-third octave sound pressure levels
determined from the output of the analyser.
3.10 Adjustments for background noise
3.10.1 Background noise shall be recorded (for at least
30 seconds) at the measurement points with the system gain
set at the levels used for the aircraft noise measurements. Thc
recorded background noise sample shall be representative of
that which exists during the test run. The recorded aircraft
noise data shall be accepted only if the background noise
levels, when analysed in the same way and quoted in PNL (see
4.1.3 a)), are at least 20 dB below the maximum PNL of the
aircraft.
3.10.2 Aircraft sound pressure levels within the 10 dBdown
points (see 4.5.1) shall exceed mean background noise
levels determined above by at least 3 dB in each one-third
octave band or be adjusted using a method similar to that
described in Appendix 3 of Doc 9501.
4. CALCULATION OF EFFECTIVE
PERCEIVED NOISE LEVEL FROM
MEASURED NOISE DATA
4.1 General
4.1.1 The basic element in the noise certification criteria
shall be the noise evaluation measure designated effectivc pcrceived
noise level, EPNL, in units of EPNdB, which is a single
number evaluator of the subjective effects of aircraft noise on
human beings. Simply stated, EPNL shall consist of instantaneous
perceived noise level, PNL, corrected for spectral
irregularities (the correction, called "tone correction factor", is
made for the maximum tone only at each increment of time)
and for duration.
4.1.2 Three basic physical properties of sound pressure
shall be measured: level, frequency distribution, and time variation.
More specifically, the ir~stantaneous sound pressure
level in each of 24 one-third octave bands of the noise shall be
required for each 500 ms increment of time during the aircraft
noise measurement.
4.1.3 The calculation procedure which utilizes physical
measurements of noise to derive the EPNL evaluation measure
of subjective response shall consist of the following five steps:
a) the 24 one-third octave bands of sound pressure level
are converted to perceived noisiness by the methods of
Section 4.7. The noy values are combined and then converted
to instantaneous perceived noise levels, PNL(k);
b) a tone correction factor, C(k), is calculated for each
spectrum to account for the subjective response to the
presence of spectral irregularities;
c) the tone correction factor is added to the perceived
noise level to obtain tone corrected perceived noise
levels, PNLT(k), at each one-half second increment of
time:
The instantaneous values of tone corrected perceived
noise level are derived and the maximum value, PNLTM,
is determined;
d) a duration correction factor, D, is computed by integration
under the curve of tone corrected perceived noise
level versus time;
e) effective perceived noise level, EPNL, is determined by
the algebraic sum of the maximum tone corrected perceived
noise level and the duration correction factor:
EPNL = PNLTM + D.
4.2 Perceived noise level
Instantaneous perceived noise levels, PNL(k), shall be calculated
from instantaneous one-third octave band sound pressure
levels, SPL(i,k), as follows:
Step I . Convert each one-third octave hand, SPL(i,k),
from 50 to 10 000 Hz, to perceived noisiness, n(i,k), by reference
to Table A4-1 (Perceived Noisiness) in Appendix 4 of
Doc 9501, or to the mathematical formulation of the noy table
given in Section 4.7.
Step 2. Combine the perceived noisiness values, n(i,k),
found in Step 1 by the following formula:
APP 2-9 24111105
Annex I6 - Environmental Protection Volume I
where n(k) is the largest of the 24 values of n(i,k), and N(k)
is the total perceived noisiness.
Step 3. Convert the total perceived noisiness, N(k), into
perceived noise level, PNL(k), by the following formula:
10
PNL(k) = 40.0 + -
log 2 1% N(k)
Note.- PNL(k) is plotted in Figure A4-1 of Appendix 4 of
Doc 9501.
4.3 Correction for spectral irregularities
4.3.1 Noise having pronounced spectral irregularities
(for example, the maximum discrete frequency components or
tones) shall be adjusted by the correction factor, C(k), calculated
as follows:
Step 1. Except for helicopters which start at 50 Hz (band
number l), start with the corrected sound pressure level in the
80 Hz one-third octave band (band number 3), calculate the
changes in sound pressure level (or "slopes") in the remainder
of the one-third octave bands as follows:
s(3,k) = no value
s(4,k) = SPL(4,k) - SPL(3.k)
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